One-side anodizing of aluminum sheet



United States Patent 3 359,190 ONE-SIDE ANODIZI G 0F ALUMINUM SHEET William Ernest Cooke, Kingston, Ontario, Canada, and Niels Otto Helwigh, San Francisco, Calif., assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada Filed Feb. 4, 1964, Ser. No. 342,451 18 Claims. (Cl. 204-28) ABSTRACT OF THE DISCLOSURE A method for continuous production of a porous anodic coating on one side only of aluminum sheet embraces advancing the sheet in submerged relation to aqueous bodies of liquid on opposite sides while said liquids are maintained in turbulent flow and anodizing current is passed from the sheet through the liquid on one side, at least said latter liquid being an anodizing electrolyte. The temperatures of the bodies of liquid are maintained at sutliciently high values for thermally conditioning the sheet and coating in a manner found necessary to prevent impairment, e.g. crazing, of the coating on subsequent high temperature sealing. Anodizing current is upwards of 100 amperes per square foot and the liquid is controlled to maintain the temperatures of the flowing bodies at sufficiently high values, of which the sum is greater than 100 C., for the stated thermal conditioning, an exemplified operation being to maintain both liquids at 65 C. The non-anodizing liquid is preferably an identical electrolyte, e.g. sulfuric acid solution, and current can be conducted to the sheet through the non-anodizing liquid, or from electrode structure in a preceding electrolyte-filled chamber through which the sheet first passes, the latter method also permitting use of alternating current for one-side anodizing.

This invention relates to the production of anodic coatings or films on aluminum, the term aluminum being employed to include aluminum base alloys which, like pure aluminum, are susceptible of being anodized to produce porous type oxide films. More particularly, the invention is directed to continuous anodizing of aluminum sheet, to produce an anodic coating on one side of such sheet in a rapid and continuous manner, especially with reference to coatings in the range of thickness of 0.1 mil and upwards, and preferably in reference to such coatings which are of porous type, e.g. as produced with sulfuric acid or equivalent electrolytes. By aluminum sheet is meant elongated aluminum pieces or material having considerable width relative to its thickness, whether designated as sheet, strip, plate, foil or the like, the method being particularly advantageous for aluminum sheet in strip form that can be coiled and withdrawn continuously from the coil through an anodizing bath, to be re-wound in coil form either before or after further treatment.

The invention is predicated on the discovery of certain new conditions or combinations of condition which cooperate in an unusual manner for the rapid attainment, in continuous operation, of a highly satisfactory anodic coating on one side of aluminum sheet material, being advantageously a porous type coating which, especially in contrast to certain very thin films of one micron or less (including certain barrier type films), has a thickness as above, of about 0.1 mil or more, and notably in such range up to 2 mils. Such anodic coating is effectively durable and adherent under many circumstances, and is adapted to serve a wide variety of finishing or protective purposes, as generally known for anodic coatings. As will be understood, these uses include protection against corrosion, wear, electrical breakdown, or other deteriorating influences, as well as the provision of 7 colored surfaces by dyeing or pigmenting the anodic film.

ance with the general understanding of the art respecting anodic coatings, they will be sometimes described herein as being oxide films, coatings or layers, i.e. aluminum oxide, although it is recognized that this is only the major component in anodic coatings.

As indicated above, the present improvements are concerned with production of a coating on one side only, of the aluminum strip. Such operation, and the resulting product, are eminently useful for many purposes, and indeed may be preferred or necessary for certain situations where aluminum sheet is employed. Furthermore, wherever a single anodized surface is adequate or desirable, there is corresponding saving in production, particularly with respect to the electrical energy required to the consumed in the electrolytic anodizing reaction.

The present procedure affords relatively high speed continuous operation, whereby unusually large current densities may be employed to achieve coatings, of desired quality and thickness (including substantial'thicknesses upwards of 0.5 mil, e.g. of the order of one mil), without diificulties or deterioration experienced in prior elforts to obtain such results. A further, specific problem that was found to arise in high-speed, one-side continuous anodizing was in maintaining the desired characteristics of the film during and after subsequent treatments such as scaling in hot water or hot aqueous solutions. Thus, Whereas it would appear that by controlling the temperature of the metal in the electrolytic anodizing stage, e.g. to a relatively low value at or approaching room temperature, best opportunity might be afforded for hastening the anodizing reaction by employment of high current densities, it was found that the film, if in the range of thickness upwards of 0.1 mil, deteriorated under sub sequent sealing operations. In particular, after the sheet had been subjected to treatment of a conventional sort, e.g. for a few minutes in water at or near boiling temperature C.), the oxide film was found to have crazed badly. This eifect of the sealing treatment, i.e. in producing a multitude of minute cracks or lines in the coating, occurred in films of various thicknesses in the stated range and was not avoided by such changes in electrolyte concentration or current density as might be achieved while still retaining a desirably high rate of production. Accordingly, a special feature of the invention'involves the discovery of procedure, particularly conditions, that effectively eliminate the tendency of the porous-type anodic coatings, formed as just described, to deteriorate, e.g. to craze, upon subsequent high temperature sealing or the like.

The objects of the invention in the production of coatings of the stated thickness, notably from 0.1 mil to 10 mils, will thus be clear from what has been said above, and include the avoidance of difficulties encountered in past attempts to achieve a desired result of high-speed, one-side anodizing. Further objects and advantages of the present improvements will likewise be apparent from the following description of the invention and of examples of its practice.

In general, the present method involves the continuous advance of the sheet through, and in effect, in submergence in aqueous electrolyte as contained in an appropriate elongated tank or vessel arranged to hold electrolyte to a level above the passing sheet. Th electrolyte is in fact arranged in two bodies, completely or at least substantially separate from each other and respectively in contact with opposite sides of the sheet, an unusually convenient arrangement being for disposition of the sheet in a horizontal position, so that the mutually isolated electrolyte bodies are respectively above and below it, although other arrangements, such as a vertical disposition of the sheet or strip, can be employed, and although in some cases (as explained below) one of the aqueous bodies here described as electrolytes may not serve an electrolytic function.

For the desired temperature control, it is important to have rapid and continuing circulation of each electrolyte immediately adjacent the surface of the traveling sheet, i.e. so that the electrolyte on each side flows continuously over the sheet, and so that the entirety of each Side of the sheet is kept in contact with the flowing liquid at all times, as by the described arrangement of submergence. In accordance with another invention, in itself described and claimed in a separate patent application, an unusually effective operation for continuously anodizing a passing aluminum surface comprises maintaining the electrolyte in turbulent flow over such surface, the flow being in a direction longitudinal of the path of the moving surface, the turbulence being substantially uniform across the surface, and the chief critical feature being that the flow is turbulent as distinguished from the kind usually called streamlined or laminar. Operation in the foregoing manner with turbulent flow has been found highly efficacious in achieving certain temperature conditions that are a basic part of the present one-side anodizing process.

With arrangements such as described above, and with the temperature of the electrolytes maintained at selected conditions of high value as explained below (conveniently at the same value on both sides of the sheet), rapid anodizing is attainable in sulfuric acid electrolyte of moderate concentration and with very substantial current densities, e.g. several hundred amperes or more per square foot. It will be understood that suitable electrodes, eg of lead as presently preferred (although other conductive bodies such as carbon, graphite, or the like, may sometimes be used), are employed, at least one in one of the two bodies of electrolyte, and preferably one in each of the bodies, at localities close to the passing strip. For instance, each electrode can consist of a plate (or an array of coplanar elements) of the conductive material, such as lead, parallel to the strip and having a plane surface of an area substantially equal to and coextensive with the adjacent exposed surface of the strip. The spacing of each electrode from the strip surface may advantageously be such as will coact in affording the desired character of electrolyte flow along the surface of the strip.

As will now be understood, the electrode on one side constitutes the cathode (being connected to the negative terminal of the source of power), so that current passes from the strip through the electrolyte to the cathode and accomplishes the anodizing action at the exposed surface of the strip. In one effective arrangement, as described above, the other electrode is disposed in the body of electrolyte on the opposite side of the strip, and serves as anode for the complete cell thus formed, i.e. so that current from the positive terminal of the source enters the second body of electrolyte from the abovementioned anode and passes to the strip, to render the latter anodic for the anodizing action. As will be explained below, alternative arrangements can be employed for conducting current to the strip, eg through an electrolytic path at another locality; in such cases the body of liquid on the side of the strip opposite to the surface being anodized may have no electrolytic function, but still, because it preferably has the same composition as the anodizing electrolyte, may be called a body of electrolyte.

As indicated above, temperature control of the opposite sides of the strip, i.e. the surface where the anodic film is forming and the opposite bare metal surface, is an important feature, such cont-r01 being realized by maintaining the temperatures of the electrolytes (moving along such surfaces) at values selected in accordance with principles that have now been discovered and that are believed to be critical for the new results desired. Thus specifically, it has been found that avoidance of crazing in a film of the stated character, under normal conditions of sealing, can be achieved by keeping the metal and film at a relatively high temperature during the anodizing step. Whereas it has heretofore been supposed that when high current densities are used and because excessive heat is deleterious to attainment of good coatings, an aluminum sheet undergoing single-surface continuous anodizing should be kept as cool as possible, it now appears that the sheet and the film being formed on it should reach a considerably elevated temperature, that this condition can only be achieved by special temperature control of the adjacent, passing electrolyte bodies as explained below, and that under such circumstances, crazing will not occur when the film is subsequently sealed.

It is believed that the exposure of anodized metal to sealant solutions at or near 100 C. tends to produce considerable tension in the oxide film, because the metal has a substantially higher coefficient of linear expansion than the film. Films such as oxide coatings, while resistant to compression stresses, are apt to be weak under tension; hence the occurrence of substantial tension in the film during sealing may result in cracking or crazing. Accordingly it was found that by allowing the temperature of the work to be very substantially raised, in effect, during the anodizing stage, so that upon cooling it outside the bath, the film might in fact be placed under compression, it was possible to avoid the crazing effect in sealing. In other words, by appropriate temperature control during anodizing, crazing or cracking can be avoided, for the reason (as now believed) that the effect of such temperature control is to afford an avoidance of undue tension in the anodic coating when the latter undergoes the sealing operation.

To the above end, it has been discovered that in a continuous one-side anodizing operation where electrolyte is rapidly, and indeed turbulently, advanced along both sides of the sheet under treatment, the temperatures of the electrolytes should be maintained at values such that as measured in degrees Centigrade, the sum of the electrolyte temperatures on the two sides is at least about 100 C. In a more specific aspect, the invention embraces the further condition that on neither side should the temperature be less than about 20 C., especially as a matter of practicality in operation. Indeed special efficiency seems to be attained where the liquid on each side is in the range of 40 C. and above, with the selected temperature on at least one of the sides at a sulficiently higher value so that the sum of the temperature is greater than 100 C. A particularly convenient mode of operation is to control the liquid temperature so that it is the same on both sides, e.g. then within a range on each side of about 50 C. and upwards, it being found that distinctly optimum results (having regard to other aspects of the anodizing treatment as Well) are achieved with each electrolyte, passing the sheet, at a temperature of about 65 C. As will be understood, higher temperatures can be used on both sides (as well as on one side) if desired, even up to 100 C., and at the latter value even on the anodically treated side in cases where chemical dissolution of the oxide coating in the electrolyte is not excessive or can be tolerated, it being presently understood such dissolution may tend to increase considerably when the temperature of a given concentration of the liquid adjacent the film being formed rises substantially above C.

Extended tests have indicated that by maintaining the liquid temperatures at selected values in accordance with the limits or ranges specified above, conditions within the work under treatment, i.e. specifically with respect to temperature conditions of the anode film as it is produced, are such that the sealing operation may be thereafter performed, at suitably elevated temperatures, without crazing or other adverse effect. Without attempting to elucidate the precise manner in which he above temperature regulation of the liquids achieves this result in the anodic coating, it may be further noted that the problem appears to be peculiar to one-side anodizing (for the production of films having at least thicknesses of the order described above), in contrast to two-side anodic treatments, where lower electrolyte temperatures can be employed without adverse consequences of the above-described sort when the anodic films are subsequently sealed in hot water or the like. An explanation of this difference may be that, in two-side operation, heating effects at the interior or base of the oxide coating are such as to maintain relatively high temperatures, there being no bare metal face for ready removal of heat by conduction. In any event, an acute problem was found to occur in high speed one-side operation, utilizing current densities of the order of 1'00 amperes per square foot, and preferably at least several hundred amperes per square foot, and also utilizing the arrangement (specially appropriate for such high current densities) of the submergence of both sides of the work in contact with rapidly traveling electrolyte. As stated, the difficulty is overcome by appropriate maintenance of the electrolyte temperatures at values selected in accordance with the above requirements, being values which presumably coact (from both sides of the sheet) in attaining the required conditions of the film for avoidance of adverse effects on subsequent high temperature sealing.

As indicated, the prescribed conditions are particularly related to operations at desirably high speeds and with the relatively large current densities needed for attainment of thick coatings at such rates of travel of the work. In consequence, the process of the invention, embracing continuous anodizing under the combination of conditions as defined, represents an improvement of significant value in the attainment of economical, continuous treatment.

For further illustration, reference may be had to the accompanying drawing, wherein:

FIG. 1 shows diagrammatically, but essentially as in vertical cross-section, an arrangement of apparatus wherein the continuous strip of aluminum is being subjected to anodization in a continuous fashion, this view also showing, as in flow sheet form, the subsequent continuous sealing of the anodized article;

FIG. 2 is a cross-section on line 22 of FIG. 1, of the anodizing tank;

FIG. 3 is a view, similar to FIG. 1, of another arrangement utilizing the invention, for anodizing one side of an aluminum strip; and

FIG. 4 is a similar view of an arrangement wherein alternating current is employed for anodic treatment in accordance with the invention.

The apparatus of the drawings, indeed shown only in a diagrammatic way, is given simply by way of illustrative example, it being understood that the equipment may vary widely in design or construction, depending on the requirements of operation, and may involve various features of structure and arrangement that will be readily apparent as appropriate to operations of the sort described. In the form of invention illustrated in FIGS. 1 and 2 of the drawings, the sheet to be anodized on one surface is represented by the aluminum strip 10, continuously withdrawn from the coil 11 and advanced (by suitable take-up means, mentioned below) lengthwise in a horizontal plane through the anodizing tank 12, which may be made of electrical insulating material, or lined with such material. As shown, the strip traverses the tank at a central or intermediate locality, i.e. between the bottom and top, so that the sheet is continuously submerged in electrolyte, constituted in effect by an upper body 14 and a lower body 15. After anodizing, the sheet is shown as traveling through the tank 16 containing sealant liquid 18, and is finally rewound as a coil 19, providing the take-up means. If desired, intermediate operations (not shown) may be performed, such as washing, dyeing, pigmenting, and indeed recoiling of the strip before it is subjected to continuous traversal of the sealant.

The tank 12 is preferably arranged so that the passing strip 10 serves as a partition between the upper and lower regions of the tank. Simply as an indication of appropri' ate mechanical means for this purpose, FIG. 2 shows a pair of resilient bearing strips 20, 21, along opposite sides of the tank, against which the edges of the strip make contact as it passes, these rubber or like elements thus maintaining mutual isolation of the two bodies of liquid 14, 15. Although it is conceivable that arrangements may be provided for entry of the strip downwardly into and upwardly out of the tank, appropriate rubber or like packing or glands 23, 24 are mounted in the end walls to provide narrow, horizontal entrance and exit slots through which .the strip may enter and leave, in liquid-tight sealed relation, thus completing the separate enclosure for the bodies of electrolyte.

For provision of current passage so as to anodize one or the other of the strip surfaces (here conveniently the upper surface), a pair of lead or other suitable electrodes 26, 27 are mounted in submerged relation in the liquids respectively with appropriate spacing, for example one inch above and one inch below the passing strip 10, each of these being simply shown as a long flat plate having a plane face parallel to and coextensive with the corresponding exposed surface of the passing aluminum sheet. -By suitable conductors 31 current is brought from the positive terminal of an appropriate D.C. source 30 to the lower electrode 27, and is returned via conductors 28 from the upper electrode 26 to the negative pole of the source. Hence the electrode 26 constitutes the cathode, and by flow of current from the sheet 10 through the electrolyte body 14 to the cathode, the upper surface of the sheet 10 is anodized. The current path to the sheet extends from the source 30, via conductors 31 to the electrode 27, and thence through the lower body of electrolyte 15 to the sheet 10, so that the latter is anodic with respect to the upper electrolyte 14, as described above.

In accordance with the invention the bodies of electrolyte above and below the aluminum sheet are caused to flow over the respective sides of the latter for the desired temperature controlling function. As diagrammatic indication of means to effectuate such circulation or flow of electrolyte, a pump 32 advances the liquid through a pipe 33 to a manifold or header 35 opening into one end of the tank 12, so that there is rapid flow of the electrolyte lengthwise of the space between the upper side of the sheet 10 and the cathode 26. The liquid leaves this region through a similar manifold 36 at the opposite end of the tank and traverses a pipe 37 to return to the pump 32. Means are provided for controlling or regulating the temperature of the aqueous electrolyte to maintain it at the desired value, in accordance with the conditions described above, as it traverses the surface of the aluminum sheet. Such means are indicated at 38, for example in the return line 37 to the pump, and may usually consist of appropriate, conventional cooling means, thermostatically controlled.

Similar liquid circulating means are shown for the lower body of electrolyte 15, i.e., a pump 42, delivering liquid through pipe 43, manifold 44, then flowing rapidly between the electrode 27 and the underside of the sheet 10, for return through the manifold 46 and pipe 47. Similar temperature control means 48 are included, eg. in the line 47 to the pump 42. Again, this temperature control instrumentality may be cooling means of a suitable conventional character, with appropriate thermostatic control. It may be noted that whereas the heating effect of the anodizing reaction is ordinarily such that both the anodizing electrolyte and the lower or coolant electrolyte will be required to perform a heat dissipating or cooling function even though the selected temperatures to be maintained at the surfaces of the sheet are relatively high,

the devices 38 and 48 may also include heating means if circumstances require. With relatively rapid fiows of electrolyte along both sides of the sheet, and appropriate avoidance of heat loss in the circulating systems, the temperature maintained in the electrolyte bodies by the instrumentalities 38, 48 will be essentially that which these liquids exhibit adjacent the surfaces of the sheet. In any event, the operation should be controlled so that, with due regard for heat losses, if any, the desired liquid temperatures are maintained along the sheet surfaces.

In this fashion, continuous and preferably rapid flow of liquid is maintained over each side of the traveling strip, advantageously in a direction countercurrent to the direction of strip advance although it is conceived that in some cases it would be feasible to provide the required characteristics of liquid travel relative to the moving strip where both are advancing in the same direction.

As explained above, it is particularly desirable that the flow be turbulent in nature, for example sufiiciently rapid under the circumstances of the fiow path, including its cross-section between the electrode and the strip, that it be turbulent rather than laminar, in accordance with established principles of flow of liquid in a conduit filled by such liquid. Indeed an important feature or element of the complete process in a specific sense is that turbulence be maintained in the electrolyte as it passes the strip, whether attained simply by selection of appropriately high velocity or with the aid of baffles or other supplemental means (not shown) of a kind conventional for promoting turbulence. It is found that by providing turbulence in the passing body of liquid, throughout its extent crosswise of the strip, efficient heat removal can be easily attained to permit maintenance of desired temperature values, with sufficient volume of electrolyte per unit time as to have no more than an insignificant tem: perature rise (e.g. one or two calities of liquid inlet and discharge.

As will be understood, ascertainment of suitable conditions, for instance a sufficiently high rate of flow, to provide turbulence is generally a matter of calculation under known principles, as by determination of flow rate of the aqueous electrolyte, for the given cross-sectional shape and size of path, which will yield a Reynolds number in the range of turbulent flow. Thus conventionally such numbers larger than about 2500 signify a condition of turbulence, it being understood that for best results in the present process, the flow conditions should be characterized by a substantially higher Reynolds number than the minimum. For example, a liquid flow affording a number of 20,000 is satisfactorily efilcient for anodizing operation at a current density of 600 amperes per square foot, and conditions of even greater turbulence, e.g. represented by Reynolds numbers as high as 100,000, can well be used, as for very high current densities. Moreover, in many cases, determination of a suitable flow rate for achieving turbulence and for maintaining a desired temperature in the passing liquid without more than a few degrees of rise can be made by simple preliminary tests and observations.

The speed of strip travel is generally governed by the thickness of film desired, having regard to the length of the anodizing path, e.g. the tank length, and the current density applied. For many purposes, speeds of the order of 1 to 20 feet per minute are suitable, in cells from one to five feet long and with current densities at the surface being anodized, in the range of 100 amperes per square foot and above, more usually several hundred amperes per square foot. In some cases, as for films in the lower part of the thickness range mentioned above, very high strip speeds can be used, i.e. up to several hundred feet per minute.

In operation of the process as exemplified in FIG. 1, the strip is continuously passed through the tank, and the effect of the current impressed in the manner described above, is to anodize the upper side of the strip, yielding degrees) between the lothe desired oxide coating. The temperature in the rapidly flowing bodies of electrolyte on both sides is carefully maintained at a selected value, for instance preferably in the range from about 50 C. to about C. These bodies of liquid are isolated from each other, especially in an electrical sense, so that there is no appreciable loss due to bypass of current through the electrolyte.

As explained above, the strip can thereafter be subjected to a conventional sealing operation, e.g. by passing it through the aqueous sealant 18 in the tank 16, at a temperature over 70 C., most usually well above 90 C., as for example at 98 C. to 100 C. The operation of sealing anodized films is well known and needs no description herein, a principal function of sealing being to reduce or eliminate the ability of the film to adsorb foreign materials, and thus to improve its properties, and indeed its appearance, as against staining or the like. Effective sealing action is obtained with conventional liquids, e.g. plain hot water, or slightly acidulated hot water having a pH in the conventional range of 5.5 to 6.5, or with special sealants such as sodium silicate or nickel acetate solutions.

By way of specific example of treatment in accordance With the invention, an aluminum sheet 10, having any desired width, say 1 foot, can be treated by traversing it through a tank where it is in submerged contact with turbu-lently flowing bodies of electrolyte 14, 15, at opposite sides, in isolation from each other. According to present indications, optimum electrolyte concentrations are sulfuric acid solutions of 15% strength, and on each side the traveling electrolyte is controlled, in rate and temperature, so that it enters the tank at approximately 65 C. and experiences no more than one or two degrees of rise in temperature after traversing the liquid-exposed surface of the sheet, while removing heat therefrom. With a suitable power source, say 35 volts D.C., an anodizing current density of 600 amperes per square foot of the exposed surface to be anodized (the upper surface in FIG. 1) is particularly suitable: in pilot plant operations the coulombic input required to produce an oxide film of 1 mil thickness was about 39,000 coulombs per square foot, representing a coulombic efficiency of about 87%. Under conditions of more ideal character, as where the speed of electrolyte travel is considerably above 2.5 feet per second, higher coulombic efficiencies are believed attainable, even up to 98% or 99%. This is in contrast to the approximately 70% efiiciency (54,000 coulombs per square foot) obtained from a number of prior operations in producing one mil thick films.

As will be understood, the required contact time for producing a film of a given thickness is determined by the given current density and the required coulombic input. Taking the values specified in the above example to yield a 1 mil film, the corresponding contact time is roughly one minute, so that in a cell 5 feet long the strip speed should be approximately 5 feet per minute. Under such circumstances, with the described electrolytes, the selected temperature of 65 C. can be effectively maintained and the desired turbulence can be achieved, in an electrolyte flowing at a speed of about 2.5 feet per second. As further example, in an unbaffled space adacent the strip surface, having a cross-section defined by a strip width of 20 inches, and a like Width of electrode spaced one inch above the strip, a flow of gallons (U.S.) of electrolyte per minute is amply turbulent and generally satisfactory.

As indicated, the treated sheet thus carries the desired anodic, oxide coating on one side, which is found to have excellent properties, e.g. in adherence, protective effect, flexibility, high porosity, ability to be dyed, and the like. It may be appropriately sealed by treatment as described above.

For other purposes, various values of concentration, temperature, current density and the like, may be selected. In general, electrolyte concentrations below 2% sulfuric acid are too low, and develop excessive heat by their resistance to current travel. Concentrations above 50% generally appear to have too much dissolving power on the oxide film, for attainment of desirably thick coatings. A more useful acid electrolyte concentration is or very preferably or more. Likewise it will ordinarily be found unnecessary to use electrolytes containing above 40% sulfuric acid. In general, of course, rapidity of action is enhanced at higher concentrations, but these involve greater limitations on the attainable film thickness, and tend to require very short times for anodizing, i.e. involving exceedingly high and impractical current densities. References herein to acid or other concentrations in solution will, of course, be understood to mean percentages by weight.

While current densities may vary from 100 amperes per square foot to 4000 amperes per square foot, the invention is of particular significance at values of 300 amperes per square foot and above, i.e. as permitting rapid anodizing action without sacrifice of desired film thickness and characteristics. Indeed values of current density of 500 amperes per square foot and higher represent a special range of departure from anything practical or feasible in prior processes of one-side D.C. anodizing. It is normally preferred to operate in the range of 600 to 1000 amperes per square foot, although current densities up to 1500 amperes per square foot are conceived as of special value. Above 4000 amperes per square foot (and to some extent above 2000), it appears that with ordinary mechanical arrangements for circulating liquid, within the space between the electrodes and the strip, it becomes more diflicult to effectuate heat removal to maintain desired temperature conditions of the work. Indeed, as indicated, a current density of 600 amperes per square foot is at present optimum, yielding a one mil film in one minute of treatment, such being fast enough to be economically feasible yet sufiiciently moderate to permit ready removal of heat.

In essentially all instances of practice of the process, relatively considerable amounts of heat are removed at both surfaces of the strip, the heat inputs being essentially equal where the temperatures of the turbulently flowing liquids are maintained at the same point.

As explained, a particular feature of the invention involves the use of liquid at the opposite or bare side of the strip which is identical with the anodizing electrolyte, viz. a sulfuric acid solution of identical concentration. While in some special circumstances acid electrolytes of different concentration may be employed, or indeed the coolant on the bare side may be an electrolyte of different composition, the mechanical nature of the apparatus is such that some minor leakage or mixing of electrolytes is almost certain to occur around the lateral edges of the passing aluminum sheet. Thus under ordinary circumstances it appears that if the concentration of acid in the electrolytes differs by more than about 1% to 2%, the anodized areas near these edges tend to have at least slightly different surface characteristics from the remainder or main area of the oxide film. Differences of this sort are particularly unsatisfactory where the film is to be dyed, since the result is a band of different shade along each edge region. Likewise, if possible, the flow rates are preferably balanced along the two surfaces, to minimize leakage.

It is conceived that some aspects of the process may be usefully applied in other circumstances, although with loss of important features and advantages of the invention. Thus it is conceivably possible, if effective isolation is achieved and minor leakage can be tolerated, to employ any liquid, such as plain water, for the coolant on the side of the strip which is not to be anodized, it being then necessary that provision be made, as with electrical contact brushes or rollers, to conduct current directly to the metallic surface of the sheet.

Alternative arrangements are also possible for effectuating the rapid, one-side anodizing treatment of the in vention while maintaining electrical connection to the strip only through electrolyte. Instead of providing electrodes in both bodies of electrolyte which are in contact with directly opposite faces of the strip, an electrode may be used only in the electrolyte on the side to be anodized. A supplemental tank or cell may then be included through which the strip passes, for example as it approaches the locality of anodizing treatment, with electrode means in such tank for travel of current through the electrolyte there to the strip.

Thus in FIG. 3 the strip 50, traveling horizontally, is arranged to pass first through a tank 51 and then through a main tank 52, with appropriate sealing means 53, 54, 55 at the vertical walls of the tanks, the latter conveniently having a common intermediate wall 56 which is provided with the seal 54 that the strip traverses. Above the strip 50 in the tank 52 there is a suitable electrode 57, of lead or other material, which provides the cathode. The tank is filled with electrolyte as indicated at 58, 59', above and below the strip and in full contact with both of its faces. The electrolyte is circulated along the faces of the strip, as described above in connection with FIG. 1, by appropriate conduit means 60, 61 for the upper surface and conduit means 62, 63 for the lower surface, it being understood that FIG. 3 is purely diagrammatic and that the crosssection of the electrolyte path below as Well as above the strip 50 may be suitably limited to permit turbulent flow of electrolyte without excess volume. Thus the process effectuated in tank 52 is exactly as in FIG. 1, except that there is no electrode in the lower liquid 59.

The preliminary tank 51 is similarly filled with electrolyte as indicated at '64, 65, conveniently both above and below the strip, with electrodes 66, 67 disposed in the respective bodies of electrolyte and in sufficiently close proximity to the strip. The electrolyte may be conveniently a sulfuric acid electrolyte, for instance identical with that employed in the tank 52 at 58 and preferably also at 59 in tank 52. The electrodes 66, 67 are connected together to the positive terminal of a suitable D.C. source 68, while the negative terminal of the source is connected to the electrode 57 in the anodizing tank 52.

Current thus flows from the source to the electrodes 66, 67 and then through electrolytes 64, 65 to the strip. Current is conducted through the metal of the strip and passes across the circulating electrolyte in the upper part of the tank 52 to the electrode 57 for return to the negative terminal of the source 68. The electrolytic treatment in the preliminary tank 51 may serve a significant cleaning function, and also provides passage of current to the strip without requiring contact brushes 0r rollers. The anodizing action occurs in the tank 52, i.e. by passage from the strip, which is anodic with respect to the electrolyte 58 and the cathode 57. Special cooling or temperature regulation may not be necessary in the preliminary tank 51. Cleaning action there (if needed) is facilitated by the employment of hot acid, and is aided to some considerable extent by hydrogen evolution at the surfaces of the strip. Oxygen would be evolved, of course, at the lead anodes in this bath, as they are connected to the positive terminal of the power supply. For some purposes the lower electrode 67 may be omitted, although it is ordinarily desirable to clean both surfaces of the strip, and with the same current density to the strip as in tank 52, the tank 51 and its electrodes may be correspondingly shorter when current passes to the strip from both sides. As stated, the anodizing process in the tank section 52 is essentially as described above in connection with FIG. 1, with corresponding maintenance of all conditions, including electrolyte flow, as there stated.

A further arragement is illustrated in FIG. 4, where the strip 70 also travels through two tanks 71, 72 in succession, each tank being arranged structurally as the tank 52 of FIG. 3, with provision for circulating electrolyte (turbulently) along both the upper and lower faces of the horizontal strip in each case and with appropriate seals for entrance and exit of the strip. Electrodes 73, 74 are respectively provided, e.g. graphite electrodes, above the strip in each tank, and are connected to the terminals of an alternating current source 75. As will be readily seen, alternating current is supplied through the electrolyte in each tank to the strip, i.e. from the electrodes, so that there is no metallic contact necessary with the strip. Anodizing action is thus performed in both tanks by the use of alternating current, in accordance with known principles of anodic treatments utilizing A.C. As in the case of the lower body of liquid 59 in FIG. 3, the liquid below the strip in each of the tanks 71 and 72 of FIG. 4 serves only as a heat exchange medium, i.e. without electrical function; although in all of these cases the lower liquid may sometimes be water or some other aqueous liquid, it is preferably an electrolyte of the same composition as the liquid above the strip, for reasons explained above.

While sulfuric acid electrolytes are particularly effective in the procedures of the invention and thus represent a cooperating feature of the treatment, other electrolytes appropriate for anodizing operations may be employed in some cases, e.g. acid electrolytes of generally equivalent function. Thus examples of suitable electrolytes for the anodic treatments here described are sulfuric acid, chromic acid, diand tri-basic organic acids, or their equivalents, either separately or in suitable combination, all as will be readily understood by persons familiar with the art of anodic treatment of aluminum to produce porous-type oxide films.

It is to be understood that the invention is not limited to the specific operations and compositions herein described but may be carried out in other ways without departure from its spirit.

We claim:

1. A method of continuously anodizing aluminum sheet to provide a porous anodic coating, on one side only, of saidsheet, comprising advancing the sheet through a treating zone while flowing first and second bodies of liquid in turbulent flow respectively over the opposite surfaces of said sheet, said first body of liquid being an anodizing electrolyte, and while passing electric current, at said treating zone, between the aluminum sheet and said first body of liquid at a density of at least 100 amperes per square foot of the surface of the sheet exposed to the first body, for anodizing said surface exposed to said first body to produce said anodic coat- 1ng, and while causing the sheet and the anodic coating thereon to reach a sufliciently elevated temperature during said anodizing operation by maintaining the temperatures of the bodies flowing over both said surfaces at sufliciently high values, of which the sum is greater than 100 C., for providing a one-side anodized sheet that is conditioned to be capable of receiving subsequent high temperature sealing treatment without impairment of the coating.

2. A method as defined in claim 1, wherein the second body of liquid is an electrolyte and the current is direct current passed through the second body of liquid to the sheet and from the sheet through the first body of liquid.

3. A method as defined in claim 1, which includes advancing the sheet through another zone in contact there with a third body of liquid which is an electrolyte, and in which the current is also passed between the sheet and said third body of liquid, whereby current enters and leaves the sheet only through electrolyte.

4. A method as defined in claim 3, wherein the current is alternating current.

5. A method as defined in claim 1, wherein the temperature in each of the bodies flowing over said sides of the sheet is maintained at a value of at least 20 C.

6. A method as defined in claim 1, wherein the temperature in each of the bodies flowing over said sides of the sheet is maintained at a value of at least 50 C.

7. A method as defined in claim 1, wherein the temperature in the bodies flowing over said sides of the sheet is maintained at a value of about 65 C. in each.

8. A method as defined in claim 2, wherein the temperature in each of said electrolytes advancing over the respective surfaces of the sheet is maintained at a value in the range extending upward from 40 C.

9. A method as defined in claim 8, wherein each electrolyte is a sulfuric acid solution having a concentration selected in the range of about 2% to about 40%.

10. A method of continuously anodizing one side only of an aluminum sheet to provide a porous anodic coating thereon, comprising advancing the sheet in submerged contact with bodies of aqueous liquid respectively at the surfaces of the sheet, at least one of said bod'es of liquid, in contact with a first surface of the sheet, being an anodizing electrolyte, while passing electric current between the sheet and said electrolyte at a density of at least amperes per square foot of said first surface, for anodizing said first surface to produce said anodic coating, continuously advancing both of said bodies of aqueous liquid in turbulent flow over the respective surfaces of the sheet in a direction aligned with the path of travel of the sheet, said flow being maintained in turbulence substantially uniformly across the sheet, and causing the sheet and the anodic coating thereon to reach a sufliciently elevated temperature during said anodizing operation by maintaining the temperatures of said bodies flowing over the respective surfaces at sufliciently high values, of which the sum is greater than 100 C., for providing a one-side anodized sheet that is conditioned to be capable of receiving subsequent high temperature sealing treatment without impairment of the coating.

11. A method as defined in claim 10, which includes passing said electric current to the sheet through electrolytic liquid to which the sheet is exposed.

12. A method as defined in claim 10, wherein said anodizing current is passed from the sheet through the electrolyte at a density of at least about 300 amperes per square foot of said first surface, to provide an anodic coating having a thickness greater than 0.1 mil, and wherein each of said bodies of liquid flowing over the surfaces is maintained at a temperature of at least about 65 C.

13. A method as defined in claim 12, wherein each of said liquid bodies is a sulfuric acid solution having a concentration of about 15%.

14. A method of continuously anodizing one side only of an aluminum sheet to provide a porous anodic coating thereon, comprising advancing the sheet in submerged contact with bodies of electrolyte respectively at the surfaces of the sheet, each electrolyte being a sulfuric acid solution selected in a concentration range of 2% to 50%, while passing electric current between the sheet and one body of electrolyte to which a first surface of the sheet is exposed, at a density of at least 100 amperes per square foot of said first surface, for anodizing said first surface to produce said anodic coating, continuously advancing both of said bodies of electrolyte in turbulent flow over the respective surfaces of the sheet in a direction aligned with the path of travel of the sheet, said flow being maintained in turbulence substantially uniformly across the sheet, and causing the sheet and the anodic coating thereon to reach a sufficiently elevated temperature during said anodizing operation by maintaining the temperatures of said bodies flowing over the respective surfaces at sufficiently high values, of which each is at least 20 C. and of which the sum is at least 100 C., for providing a one- 13 side anodized sheet that is conditioned to be capable of receiving subsequent high temperature sealing treatment without impairment of the coating.

15. A method as defined in claim 14, wherein said electrolytes are of identical concentration, selected in the range to 40%.

16. A method as defined in claim 15, which includes passing said electric current to the sheet through the body of electrolyte which is in contact with the surface of the sheet opposite to said first surface.

17. In a method of finishing an aluminum sheet which includes continuously anodizing one side only thereof, the steps of advancing the sheet through a treating zone while flowing first and second bodies of liquid in turbulent flow respectively over the opposite surfaces of said sheet, said first body of liquid being an anodizing electrolyte, and while passing electric current, at said treating zone, between the aluminum sheet and said first body of liquid for anodizing the surface of said sheet exposed to said first body, at a current density of at least 100 amperes per square foot of said last-mentioned surface, to produce an oxide film constituting a porous anodic coating thereon, and thereafter subjecting the anodized aluminum sheet to treatment in hot aqueous liquid for sealing said oxide film, said steps of flowing said bodies of liquid over the sheet in the treating zone including maintaining the temperatures of said bodies flowing over said surfaces at values of which the sum is at least 100 C., for so thermally conditioning the sheet during anodizing, as to prevent impairment of said film by said hot sealing.

18. In a method of finishing an aluminum sheet which includes continuously anodizing one side only thereof, the steps of advancing the sheet in submerged contact with bodies of substantially identical electrolyte respectively at the surfaces of the sheet, such electrolyte being a sulfuric acid solution selected in a concentration range of 2% to 50%, while passing electric current between the sheet and one body of electrolyte to which a first surface of the sheet is exposed, and across said last-mentioned electrolyte body to an electrode surface substantially coextensive with said exposed first sheet surface, at a density of at least 100 amperes per square foot of said first surface, for anodizing said first surface to produce an oxide film thereon having a thickness of at least 0.1 mil, and While continuously advancing both of said bodies of electrolyte in turbulent flow over the respective surfaces of the sheet in a direction aligned with the path of travel of the sheet, said flow being maintained in turbulence substantially uniformly across the sheet, and thereafter subjecting the anodized aluminum sheet to treatment in aqueous liquid at a temperature of at least C. for sealing said oxide film, said steps of advancing said bodies of electrolyte over the sheet while passing said current including maintaining the temperatures of said bodies in turbulent flow over said surfaces at values which are each in the range of 20 C. to C. and of which the sum is greater than 100 C., for so thermally conditioning the sheet during anodizing, as to prevent impairment of said film by said high temperature sealing.

References Cited UNITED STATES PATENTS 2,370,973 3/1945 Lang 204-28 2,930,739 3/1960 Burnham 204-28 2,989,445 6/ 1961 Lloyd et a1. 20428 3,079,308 2/ 1966 Ramirez et al 20428 FOREIGN PATENTS 608,557 9/1948 Great Britain.

ROBERT K. MIHALEK, Primary Examiner.

JOHN H. MACK, HOWARD S. WILLIAMS,

Examiners.

T. TUFARIELLO, Assistant Examiner, 

1. A METHOD OF CONTINUOUSLY ANODIZING ALUMINUM SHEET TO PROVIDE A POROUS ANODIC COATING, ON ONE SIDE ONLY, OF SAID SHEET, COMPRISING ADVANCING THE SHEET THROUGH A TREATING ZONE WHILEFLOWING FIRST AND SECOND BODIES OF LIQUID IN TURBULENT FLOW RESPECTIVELY OVER THE OPPOSITE SURFACES OF SAID SHEEET, SAID FIRST BODY OF LIQUID BEING AN ANODIZING ELECTROLYTE, AND WHILE PASSING ELECTRIC CURRENT, AT SAID TREATING ZONE, BETWEEN THE ALUMINUM SHEET AND SAID FIRST BODY OF LIQUID AT A DENSITY OF A LEAST 100 AMPERES PER SQUARE FOOT OF THE SURFACE OF THE SHEET EXPOSED TO THE FIRST BODY, FOR ANODIZING SAID SURFACE EXPOSED TO SAID FIRST BODY TO PRODUCE SAID ANODIC COAT- 